Add randomizing gravity boot laser beam color

[bismurphy]

This is my first contribution to this repo, so please do tell me if I'm doing anything out of line :) I didn't see any guidelines on code style or similar, so hopefully what I have is okay, but I'm more than happy to make any requested changes.

This randomizes the color of the "laser beam" that comes out of Alucard's feet when using the gravity boots. This is similar to the existing cloak color randomization.

The logic here is fairly simple and seems to work well. Once players have tried this out, the logic could be tweaked to curate colors, but I think for a start this is a good system.

I chose to put the call to the new color randomizing function in the same location as the call to cloak randomization, since they seem to be along the same theme.

Excited to contribute to the randomizer!